In recent years, high-pressure fuel injection apparatuses capable of injecting high-pressure fuel directly into the cylinders of an internal combustion engine have been employed practically. In conventional high-pressure fuel injection apparatuses of this type, high-pressure fuel obtained by use of a high-pressure pump is accumulated in a common rail and the required amount of high-pressure fuel is directly injected from the common rail into the cylinders. The valve-open period of each injector is controlled by adjusting the pulse width of a drive pulse for driving the injector in accordance with a target amount of fuel injection determined on the basis of the operation condition of the engine at each instant. Consequently, it follows that the amount of fuel injected into the cylinder through an injector when opened is affected by the pressure of the fuel at that time.
To overcome this disadvantage, JP A Hei 7-269394 teaches an arrangement in which the pulse width of the drive pulse signal (injection pulse width) for determining the actual injection period of the injector needed at each instant is corrected in accordance with the actual pressure of the fuel supplied to the injector so as to obtain the desired amount of fuel injection regardless of variation in the pressure of fuel supplied.
The functional relationship established between the injection period of the injector and the amount of fuel injected through fuel pressure as a parameter is utilized in the known apparatus disclosed in JP A Hei 7-269394. This conventional apparatus is arranged in such a way that the fuel pressure at that time is measured prior to the fuel injection and the injection period of the injector is corrected by use of the functional relationship and the measured result, whereby the desired amount of fuel injection can be obtained regardless of variation in the pressure of fuel supplied.
When the arrangement described above is employed, the target amount of fuel injection can be reliably obtained in the case where the fuel pressure stays at the previously measured level during the period of fuel injection. However, the pressure of fuel accumulated in the common rail of an actual apparatus decreases when the fuel is injected from the injector. Therefore, according to the foregoing conventional technology in which the injection period of the fuel from the injector is determined on the basis of only the fuel pressure detected previously, the actual amount of fuel injected becomes less than the target amount of fuel injection and accurate fuel injection operation cannot be achieved.
It is an object of the present invention to provide a method and apparatus for injecting high-pressure fuel which is capable of overcoming the disadvantages mentioned above.
It is another object of the present invention to provide a method and apparatus which is capable of precisely and economically controlling the amount of fuel injected with consideration to decrease in fuel pressure caused by fuel injection.